Secret-communication system



June 17 1924. 1,497,714

L. ESPENSCHIED sacaa'r comaumcnmu SYSTEM Filed Jan. 28, 1921 Band filter alten uatqs 600-900- .Band Filter e allenuate Zoo-600'" I? Z a); 6' 0M 7 f 4 .7 V w Band Fi/fer attenuates 900- Band Fidel attenuaje's 1400-2000 50 cluding the baud filters BF, and BF.

Patented June 1 7, 1924.

UNITED STATES PATENT OFFICE.

LLOYD ESPENSCHIED, O'F QUEENS, YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY; A CORPORATION OF NEW YORK.

SECRET COMMUNICATION SYSTEM.

Application filed January 28, 1921. Serial No. 440,679.

To all whom it may concern:

Be it known that I, Lnoro ESPENBCIIIED, residing at Queens, in the county of Queens and State of New York, have invented cer- 5 tai Improvements in Secret-Communicatioab ystems, of which the following is a speci cation.

Th invention ielatcs to systems of communir iition and more particularly to arrangements for providing secrecy in the transmission of messages over said systems.

In the arrangements of this invention secrecy is obtained by dividing the messa e frequency band into a plurality of so 5 bands and subjecting said sub-bands to differentdegrees of. attenuation before transmission. Aceordin ly when the .message frequency band is t us transmitted it Wlll be s6,;distorted that it will be practically unintelligible to anyone who might-have unauthorized access to the fluedium through or overwhich the mcssag was being transmitted. To render the message'intelligible at the receiving station the sub-bands of the 2 message frequency band are again subjected to unequal degrees of attenuation, so that all of the subbands will be attenuated to the same degree and the distortion thus 're-' mujed. The message frequency band may 'tb n be amplified and transmitted to receivi arrangements 'in intelligible form.

' 'he invention may be more fully understood from the following description, when read in connection with the accompanying drawing, in the Figures 1, 2, 3 and 4 of which are illustrated arrangements embodying a preferred form of theinvention. In Figs. l and 2 respectively are shown the transmitting and receiving arrangements of 40 the invention, while in Figs. 3 and 4: are

shown graphical representations of the transmission frequency characteristics of the message frequency band as pruluccd by the. transmitting and receiving arrangements of the invention.

In the transmitting arrangcmcntsof Fig. 1 there is shown a circuit including a trans mitter 1. This transmitter is coupled through the transformer 2 to a circuit in- These band filters may be of'the type illustrated in an article entitled Theory and design of uniform and composite electric wave filters by O. J. Zobel, published in the Bell System .baud filter BF, might attenuate aband of frequencies from 1100 to 1400 aud readily transmit other frequencies. With this arrangement it will be seen that certain of the sub-bands of, the message frequency band, such as sub-bands 600 to 900 and 1100 to 1400, will be subjected to a greater degree of attenuation than the other sub-bands of the message. It will be seen from Fig. 3- that the transmission-volume-freq\iency characteristics of the message, which if-the band filters were omitted would be slow dotted line PQ, will assume the form of..:tlie curve. C, when the aforementioned sub-bands are attenuated by the filters to the dotted line XY. Accordingly hy thiis subjecting certain of the sub-bands to a attenuation than others, t to message frequency band may be so distorted before transmission as to render it unintelligible. The circuit including the band filters is coupled through transformer 3 to a circuit including a modulating device M whereby the messa e frequency may be impressed on a high requeucy carrier wave. Any desirable form of modulating arrangement may be-utilized in this connection, the arrangement herein shown bei mg the well known duplex vacuum modulator illustrated the U. 5. Patent No. 1,343,307, issued June 55, 1920, to J. R. Carson. The output circuit of the modulator M may be coupled through a transformer-4 to a circuit includin an amplifying device. The amplifier A, ierein illustrated, is of the well known vacuum bulb type and no further description thereof will be given. The output circuit of the amplifier A may be coupled through a transformer 5 to a transmitting channel L. from which the message may be transmitted over a wire line or may be modulated upon a high frequency carrier, if desired, and then transmitted either'by wire or radio. Obviously other reater degree .of

forms of transmitting circuits might be coupled to the. band filter circuit rather than the one shown, which is illustrative only.

In the receivin r arrangements of the invention illustrator in Fig. 2 thejncoming port-ion of the transmitting channel Ljs shown coupled through a transformer- 6 toa circuit including a demodulating device DM; which may be of the well known dulex vacuum type illustrated in the U; -S. Extent, No. 1,343,308 issued June 1.5, 1920 to J. R. Carson. The output circuit of the dc modulator DM is coupled through atran'sformer 7 to a circuit including :the' band filters BF BF, and BF filters may be of the typei llustratediri the aforementioned article by O. J. Zobel. Band filters BF, may be adapted to attenuate 'a seen from Fig. 4 that the normal message frequency band, which would ha ve an atsub-band of frequencies between-200 and 600 cycles, while-band filters BF, will attenuate a sub-band between '900 and 1100 and band filter BF will attenuate a sub-band' between. 1400 and 2000 cycles. These band'filters should be adjusted to subject-these "sub bands to the "same degree of attenuation'as the band filter BF and BF, jIt will be tenuation frequency characteristic indicated by the dotted line PQ', would when transmitted through the band filters BF, B11,

and BF have an attenuation frequency characteristic as shown by the curve CC The component of the curve C of Fig.3 and the curve C of Fig. 4 would however be represented by the dotted line XY. In 0t er words when the distorted messagefrequency band, which'comes in over. the cham nel L, is transmitted through the band filtors BF,, BF, and BF}, all of the sub-Hands will have been subjected to the same degree of attenuation and restored to the same, though lower, level in the attenuation scale. This will serve to remove the distortion from the messa e frequency 'band and to deemed, desirableit is understood that the Theseband= The arrangements are illustrative only. The bandfilters, for example, might be increased in number and adjusted to operate with other sub-bands than those shown. Accordingly, it is understood that the arrangements of the invention are capable of' embodiment in many other and widely varied foi'mswithout departing from the spirit of the invention as defined claims.

-WJiat is claimed is:

1. Ina transmission system including a in the appended sending station and a receiving station the.

method of maintaining secrecy in the transniision of .messagesbetween. said stations which consists in distorting the outgoing message currents by subjecting certainof the frequency bands of said currents toa certain degree of attenuation, andin re s toi'ing the am litude relation of the several frequency ban of said message-currents to normal at the receiving station by subjecting otlier of the frequency bands of the incoming message vcurrents to a similar degree of attenuationand amplifying all of I said bands.

'2. In'a transmission system including a sending station anda' receiving station' the -m etho( l of maintainin secrecy in the transm ssion of messages etween said stations which consists in distorting. the outgoing message currents by subjecting certain of the frequency bands of said currents to a -certain degree of attenuation, and in restoring the relativeamplitude relation of the several frequency bands of said message currents at the receiving station by subjecting other of the frequency bands of the incoming message currents to a similar degree of attenuation.

'3.'A transmission system including a sending station and a receivin station, a transmitting circuit at said sen in station, means for selecting certain of the requency bandsfiransmitted over said circuit and for subjecting said selected bands to a greater degree of attenuation than the other of said bands, means for transmitting all .of said frequency' bands to said rec ving station, means at said receiving station for subjecting the other of said bandsto the same degree of attenuation as said selected bands, and means at said receiving station for V am lifying all ofsaid bands.

11 testimony whereof, have signed name to this specification this 25th day of J anuary 1921.

: LLOYD ESPENSCHIED. 

